The long-term goal of Peripheral Mechanisms of Hearing is to describe the transformations that acoustic signals undergo in the ear, focusing on the vibrations of the basilar membrane, the mechanical analyzer that separates sound into its frequency components and on which rests the organ of Corti and its hair cells, which convert vibrations into bioelectric potentials; the trains of action potentials that travel along auditory-nerve fibers, carrying acoustic information to the brain; and otoacoustic emissions, sounds produced by the ears of four-limbed animals, including humans. The investigations are carried out in deeply-anesthetized chinchillas, gerbils and pigeons. The ears of chinchillas and gerbils resemble those of other mammals including humans, and therefore serve as models to study cochlear function of direct relevance to human hearing. Birds, including pigeons, are more distantly related to humans, having evolved from dinosaurs. The study of their ears serves as a comparative counterpoint to the studies in chinchilla and gerbil. Cochlear vibrations are recorded with a laser system capable of measuring displacements of atomic dimensions. Otoacoustic emissions are measured with a sensitive microphone. The electrical responses of individual auditory-nerve fibers are recorded with fine-tipped microelectrodes. Cochlear vibrations will be measured at both apical and basal regions, which principally encode low- and high-frequency sounds, respectively. The responses of auditory-nerve fibers will be studied as functions of stimulus frequency and intensity, to establish their correspondence with the underlying vibrations. Otoacoustic emissions will be measured in animals with intact ears and also in conjunction with simultaneous recordings of cochlear vibrations or responses of auditory-nerve fibers. Similar studies will also be carried in pigeons.